I mean look, He is obviously a creepy old man who is grooming her.
Him talking about his evil wife is what abusers do. They have to make it seem like they have an excuse for trying it on with you because their wife is evil incarnate. Dirty old man.
And of course he will be driving a wedge between you.
I’d tell her that you respect her wish to end things and wish her all the best but, that she is being groomed and would be wise to read up on grooming and how it takes place and how to spot narcissist love bombing and be careful.
Something like “I respect your decision to end things and wish you well. But please read up on grooming and love bombing and how to spot abuse. Please be aware that a man who talks horribly about his wife, is doing so to con you into thinking it’s ok to cheat with him. His wife is probably a lovely woman. And he, is a dirty old man. Stay safe Sarah, wishing you all the best”.
And then once she has read it, block her and start moving on with your life.
Befuae yes when people we love are in danger we should warn them. Unfortunately, this narcissistic man has got his claws in so she may not listen. But maybe in future when she starts to realise he is bad news, she will google grooming and start to feel more confident to leave him. So you will have at least helped there.
But the more you push, the more she will pull away. And, frankly, you two are finished now, reasons aside. So make a clean break and start focusing on getting over things yourself. Warn her, then block her. That way you can rest easy knowing you did what you can. But are still respecting the break up. Because regardless of the reason, no means no. And you should respect her boundaries. I’m sorry, break ups are hard. But hopefully her family will help her if she needs to escape him. You just focus on taking care of your mental health and getting over the break up.